Absolute rate constants for SN1-like ionization reactions in zeolites: Determination of zeolite ionizing power

Article abstract of DOI:10.1021/ol990728o

(Formula presented) A kinetic probe molecule has been developed to investigate the absolute ionizing power of cation-exchanged (LiY, NaY, KY, RbY, and CsY) Y-zeolites. The probe, the 2-chloro-1-(4-methoxyphenyl)ehtyl radical, was generated within the supe

Full text of DOI:10.1021/ol990728o

ORGANIC
LETTERS
1999
Vol. 1, No. 3
531-534
Absolute Rate Constants for SN1-like
Ionization Reactions in Zeolites:
Determination of Zeolite Ionizing Power
Wendy Ortiz, Frances L. Cozens,* and Norman P. Schepp
Department of Chemistry, Dalhousie UniVersity, NoVa Scotia, Canada, B3H 4J3
fcozens@is.dal.ca
Received June 10, 1999
ABSTRACT
A kinetic probe molecule has been developed to investigate the absolute ionizing power of cation-exchanged (LiY, NaY, KY, RbY, and CsY)
Y-zeolites. The probe, the 2-chloro-1-(4-methoxyphenyl)ethyl radical, was generated within the supercages of MY upon laser photolysis of
2-chloro-1-(4-methoxyphenyl)ethyl acetate under both dry and hydrated conditions. The ionizing power of the cation-exchanged Y-zeolites
depends on the nature of the counterion and increases upon going from CsY to LiY. In addition, the ionizing powers of the dry zeolites are
weaker than that of neat water and are more similar to water/methanol mixtures containing from 30% to 70% water. A significant reduction in
the absolute ionizing ability of the zeolite was found when the zeolite was hydrated by the coadsorption of water.
Zeolites are microporous crystalline aluminosilicate materials
made up of [SiO4]4- and [AlO4]5- tetrahedra where every
aluminum present in the framework carries a net negative
charge that must be balanced with a charge-balancing
cation.1,2 The tetrahedra are arranged to give zeolites an open
framework structure consisting of cavities and pores that
readily accept organic compounds. The internal properties
of zeolites and their effect on the chemistry of incorporated
guest molecules have been the subject of extensive studies.3-8
One well-documented property of their internal voids is the
ability to support and promote charge separation reactions
such as, for example, oxidation of alkenes to radical
cations9-11 and bond heterolysis of alkyl halides to carboca-
tions.12,13 To fully understand these reactions taking place
within zeolites, it would be useful to have quantitative
information concerning the ionizing ability of various zeo-
lites, especially in comparison to that of common solvents
such as water or alcohols. For example, with respect to bond
heterolysis reactions, knowing where the ionizing power of
zeolites ranks on scales that are commonly used to represent
the ionizing power of solvents and solvent mixtures, such
as the Y scale,14,15 would be useful in the further development
of zeolites as host materials for organic reactions.16
In the present work, we focus on determining the ionizing
power of various alkali metal cation exchanged Y-zeolites
(9) Pitchumani, K.; Corbin, D. R.; Ramamurthy, V. J. Am. Chem. Soc.
1996, 118, 8152-8153.
(10) Cozens, F. L.; Bogdanova, R.; Re´gimbald, M.; Garc´ıa, H.; Mart´ı,
V.; Scaiano, J. C. J. Phys. Chem. 1997, 101, 6927-6928.
(11) Yoon, K. B. Chem. ReV. 1993, 93, 321-339.
(12) Murray, D. K.; Chang, J. W.; Haw, J. F. J. Am. Chem. Soc. 1993,
115, 4732-4741.
(13) Murray, D. K.; Howard, T.; Goguen, P. W.; Krawietz, T. R.; Haw,
J. F. J. Am. Chem. Soc. 1994, 116, 6354-6360.
(14) Bentley, T. W.; Llewellyn, G. Yx Scales of SolVent Ionizing Power;
Taft, R. W., Ed.; J. Wiley & Sons: New York, 1990; Vol. 12, pp 121-
158.
(15) Fainberg, A. H.; Winstein, S. J. Am. Chem. Soc. 1956, 78, 2770-
2777.
(1) van Bekkum, H.; Flanigen, E. M.; Jansen, J. C. Introduction to Zeolite
Science and Practice; Elsevier: Amsterdam, 1991.
(2) Breck, D. W. Zeolite Molecular SieVes; John Wiley and Sons: New
York, 1974.
(3) Ward, J. W. J. Catal. 1968, 10, 34-46.
(4) Corma, A. Chem. ReV. 1995, 95, 559-614.
(5) Rao, V. J.; Perlstein, D. L.; Robbins, R. J.; Lakshminarasimhan, H.
M.; Kao, H. M.; Grey, C. P.; Ramamurthy, V. Chem. Commun. 1998, 269-
270.
(6) Hattori, H. Chem. ReV. 1995, 95, 537-558.
(7) Lavelley, J. C.; Lamotte, J.; Travert, A.; Czyzniewska, J.; Ziolek,
M. J. Chem. Soc., Faraday Trans. 1998, 94, 331-335.
(8) Ramamurthy, V. Photochemistry in Organized and Constrained
Media; VCH: New York, 1991.
(16) (a) Conceptually similar attempts have been made to place the acid
strength of zeolites on scales such as the Hammett acidity function, Ho. (b)
Haw, J. F.; Nicholas, J. B.; Xu, T.; Beck, L. W.; Ferguson, D. B. Acc.
Chem. Res. 1995, 29, 259-267.
10.1021/ol990728o CCC: $18.00 © 1999 American Chemical Society
Published on Web 07/01/1999

by using nanosecond diffuse reflectance laser photolysis to
measure quantitatively the absolute rate constants for the
heterolytic cleavage of a carbon-leaving group bond, a
reaction that is analogous to the rate-determining step of
fundamental SN1 reactions. Our approach involved the
development of a kinetic probe that is specifically designed
to undergo a rapid SN1-like ionization reaction with a rate
constant that is sensitive to its environment. The organic
probe, the 2-chloro-1-(4-methoxyphenyl)ethyl radical 2, eq
1, has several properties that make it ideal for examining
Figure 1. Diffuse reflectance spectra obtained after 266-nm laser
photolysis of 1 in hydrated RbY under vacuum (10-3 Torr)
conditions. Spectra were obtained 520 ns (0), 1.80 µs (O), and
12.9 µs (9) after the laser pulse. The inset shows the change in
reflectance at 610 nm as a function of time upon 266-nm laser
photolysis of 1 in dry NaY(0), RbY(b), and CsY(4) at 22 ( 1
°C.
the ionizing power of zeolites. The radical is readily
generated photochemically from a precursor, 2-chloro-1-(4-
methoxyphenyl)ethyl acetate 1, which is thermally stable
within zeolites.17 In addition, the product formed upon
ionization of the carbon-chlorine (C-Cl) bond is the
4-methoxystyrene radical cation 3, which has distinctive
absorption bands at 365 and 610 nm,18 and is relatively long-
lived within zeolites.10 As a result, the ionization reaction is
easily monitored using time-resolved diffuse reflectance
spectroscopy. Furthermore, the reactivity of 2 in solution has
recently been studied,19 and these results can be applied to
determine the ionizing power of zeolites relative to common
solvents such as water and 2,2,2-trifluoroethanol (TFE).
Finally, the size and shape of the probe molecule is similar
to that of many classes of aromatic compounds that are
currently being investigated within the cavities of zeolites.
Transient diffuse reflectance spectra generated upon 266-
nm laser irradiation20 of 2-chloro-1-(4-methoxyphenyl)ethyl
acetate 1 under vacuum (10-3 Torr) conditions in dry and
slightly hydrated21 (5 ( 1 wt %) alkali metal cation
exchanged zeolites22 (LiY, NaY, KY, RbY, and CsY)23 are
in each case dominated by strong absorption bands at 365
and 610 nm. Figure 1 shows the spectra obtained after 266-
nm laser excitation of 1 in slightly hydrated RbY as a
representative example. The bands at 365 and 610 nm are
identical to those for the 4-methoxystyrene radical cation
generated previously in solution18 and in zeolites,10 and can
therefore be confidently assigned to the 4-methoxystyrene
radical cation 3. The spectrum also shows the presence of a
transient species at 310 nm, which is similar to that of radical
2 generated upon laser photolysis of 1 in solution.19 The
transient is completely quenched by oxygen, confirming its
identity as radical 2. In the absence of oxygen, the radical
decays with a first-order rate constant of 6 × 105 s-1 which
matches the growth of the absorption bands at 365 and 610
nm due to the formation of radical cation 3. This indicates
that the radical cation is generated by unimolecular heteroly-
sis24 of the C-Cl bond (eq 1). Support for this conclusion
comes from the observation that purging the zeolite sample
no water was coabsorbed during the transfer. The samples were then placed
back on the vacuum line for at least 5 h and sealed under vacuum (10-3
Torr). The loading level was kept constant at one molecule of substrate for
every 10 zeolite cavities.
(23) (a) The exchanged zeolites were prepared by stirring NaY with 1
M aqueous solutions of the corresponding chlorides (LiCl, KCl, RbCl, CsCl)
at 80 °C for 1 h. The zeolites were then washed until no chlorides appeared
in the washing water and dried under vacuum. This procedure was repeated
three times and the zeolite was calcinated between washings. NaY has three
types of exchangeable cations. The number of cations per unit cell of zeolite
Y are 16 site I cations, 32 site II cations, and 8 site III cations. It is well
established that only site II and III cations are accessible to guest
molecules.23b The percent exchange was found to be 47% for LiY, 97%
for KY, 44% for RbY, and 47% for CsY. It is known that for the larger
cations such as Rb+ and Cs+ only the accessible Na+ which occupy type
II and type III sites can be readily exchanged.1 Thus, the maximum cation
exchange is ∼70%. Values of ∼50% exchange indicate that a small
percentage of type II cations are not completely exchanged. The low percent
exchange for the LiY sample used in this study is because hydrated Li+
also does not readily exchange the type III cations. (b) Ramamurthy, V.;
Turro, N. J. J. Inclusion Phenom. Mol. Recogognit. Chem. 1995, 21, 239-
282.
(17) Continuous extraction of zeolites after incorporation of 1 using
hexane as the solvent resulted in complete recovery of 1.
(18) Schepp, N. P.; Johnston, L. J. J. Am. Chem. Soc. 1993, 115, 6564-
6571.
(19) Cozens, F. L.; O’Neill, M.; Bogdanova, R.; Schepp, N. P. J. Am.
Chem. Soc. 1997, 119, 10652-10659.
(20) Continuum Nd:YAG laser 266 nm, <8 mJ/pulse, <8 ns/pulse.
(21) Water was introduced by cooling the activated zeolites under
atmospheric conditions for a specific time period (∼1 min) prior to sample
incorporation.
(22) The Y-zeolites were activated at 400 °C for 24 h, except LiY which
was activated at 500 °C. Substrate 1 was incorporated by stirring ∼3.5 mg
of 1 dissolved in 20 mL of hexane with ∼350 mg of activated zeolite for
1 h. The suspension was centrifuged, and the isolated zeolite was washed
with hexane and dried under vacuum for at least 12 h. The dried samples
were then transferred into laser cells in a nitrogen glovebag to ensure that
(24) Direct detection of the formation of the radical cation from the
initially generated radical unambiguously confirms that the ionization
reaction is unimolecular and is not assisted by the zeolite framework through
nucleophilic assistance.
532
Org. Lett., Vol. 1, No. 3, 1999

with oxygen completely inhibits the formation of the radical
cation. As the 4-methoxystyrene radical cation is largely
unreactive toward oxygen,18,25 the nonappearance of radical
cation 3 under oxygen conditions26 is best explained by a
process in which the radical is trapped by oxygen more
rapidly than ionization of the leaving group.
Kinetic traces for the C-Cl bond heterolysis as monitored
by the formation of the radical cation at 365 or 610 nm were
obtained in each of the different cation-exchanged Y-zeolites
under dry and slightly hydrated conditions, and representative
results are shown in Figure 1. In each case, the growths fit
well to a first-order expression, giving the rate constants
summarized in Table 1.27
Figure 2. Schematic representation of the ionizing power of dry
alkali metal cation exchanged Y-zeolites relative to the ionizing
power of common solvents.
1.8 × 106 s-1 to 3.9 × 107 s-1 and are considerably smaller
than the rate constant of >1 × 108 s-1 for the same reaction
in the highly ionizing solvents, water and 1,1,1,3,3,3-
hexafluoro-2-propanol (HFIP). Thus, while these alkali metal
cation exchanged Y-zeolites clearly support bond heterolysis
reactions, their ionizing powers are at least 1 to 2 orders of
magnitude weaker than those of strongly ionizing solvents.
On the other hand, the zeolites possess an ionizing power 1
to 2 orders of magnitude stronger than that of neat methanol
in which the heterolysis reaction of 2 is too slow (<105 s-1)
to observe. The ionizing power of NaY is also somewhat
stronger than that of TFE, another common solvent that
enhances solvolysis reactions, while KY has an ionizing
power that is virtually identical to that of TFE.
Estimates of Y values for the alkali metal cation exchanged
zeolites can be made by placing the rate constants measured
in the present work on the correlation line defined by the
relationship between rate constants for heterolysis of 2 in
methanol/water mixtures19 and the corresponding YAdCl
values.14 The Y values range from 1.8 for CsY to 4.5 for
NaY under dry conditions (Table 1), indicating that CsY has
an ionizing power similar to that of 30% aqueous methanol,
while the ionizing power of NaY is similar to that of 70%
aqueous methanol.
Table 1. Rate Constants for the Heterolysis of Chloride from
the 2-Chloro-1-(4-methoxyphenyl)ethyl Radical (22 ( 1 °C) in
Dry and Hydrated Alkali Metal Cation Exchanged Y-Zeolites
a
khet
,
s-1
fieldb
V Å-1
Y values
(dry)c
M
dry
hydrated
Li+
>50 × 106
>50 × 106
2.1
1.3
1.0
0.8
0.6
>4.7 (7.3)d
Na+ (39 ( 2) × 106
(12 ( 3) × 106
4.5
3.3
2.5
1.8
K+
(9.9 ( 0.5) × 106 (1.8 ( 0.5) × 106
Rb+ (4.0 ( 0.6) × 106 (0.6 ( 0.1) × 106
Cs+ (1.8 ( 0.3) × 106 (0.6 ( 0.1) × 106
a Average of 2-4 rate constants from time-dependent diffuse reflectance
changes at 365 and 610 nm. b Electrostatic field strength in dry zeolites
taken from ref 3. c Y values calculated from khet (dry) values as explained
in the text. d The Y value of 7.3 for dry LiY is estimated using the
extrapolated value, khet ) 1 × 109 s-1 (ref 27).
These rate constants provide interesting insights into the
ionizing power of zeolites felt by the probe molecule. First,
the rate constants for heterolysis are all quite large and
illustrate very clearly that the environment within the zeolites
is well-suited to support reactions involving transition states
with significant charge-separation characteristics. In addition,
the rate constants increase considerably as the size of the
counterion decreases. This trend follows the increasing
electrostatic field strength within zeolites as the size of the
counterion decreases28 (Table 1) and shows that electrostatic
field strength is an important parameter in determining zeolite
ionizing power. Our results also show that the rate constants
are highly sensitive toward the hydration state of the zeolite.
In particular, the ionization reaction in the slightly hydrated
zeolites is consistently slower than in the corresponding dry
zeolites. Presumably, the water molecules associate with the
cations and attenuate the electrostatic field,29 reducing the
ionizing power and causing a decrease in the rate constant.
It is especially interesting to compare the results in the
present work to the rate constants for the same heterolysis
reaction measured previously19 in solution. As shown in
Figure 2, the rate constants in the dry zeolites range from
Just as Y values in solution are not absolute and depend
on a variety of factors, including the nature of the leaving
group, the ionizing powers measured in the present work
may not be universally applied to all substrates. Nonetheless,
the fact that the structural features of the probe molecule
which include an aromatic ring and chloride ion as the
leaving group are common in organic systems indicates that
our results should be relevant to a wide range of organic
substrates. In addition, the close similarity between our
(27) Strong, long-lived (τ ) 20 ns) fluorescence at 365 nm was observed
in LiY, making it impossible to monitor the real growth of the radical cation.
However, at 610 nm, the growth was essentially complete within the laser
pulse, which leads to a lower limit estimate of 50 × 106 s-1 for the
heterolysis of the C-Cl bond of radical 2 in LiY. A much larger rate
constant of 1 × 109 s-1 is predicted by extrapolation of the linear relationship
between the log of the rate constants in dry CsY, RbY, KY, and NaY and
electrostatic field strength.
(28) (a) Other factors could contribute to the observed reactivity trend
such as enhanced electrophilic catalysis by the smaller alkali metal cations.
However, while electrophilic catalysis by some metal cations such as Ag+
and Hg2+ has been observed in solution for the ionization of chloride from
alkyl chlorides, alkali metal cations show no such activity. (b) Clacke, G.
A.; Taft, R. W. J. Am. Chem. Soc. 1962, 84, 2295-2303.
(25) It is well-documented that cationic species typically do not react
with oxygen.
(26) The sample was purged with dry oxygen for 30 min prior to
photolysis.
(29) Imanaka, T.; Okamoto, Y.; Takahata, K.; Teranishi, S. Bull. Chem.
Soc. Jpn. 1972, 45, 366-372.
Org. Lett., Vol. 1, No. 3, 1999
533

observation that the internal cavities of zeolites possess an
ionizing power that is analogous to solvents such as aqueous
methanol and TFE, and conclusions from previous work
using aromatic probes that zeolites are quite polar30,31 with
intracavity micropolarities close that of 50% aqueous metha-
nol32 and Taft R values similar to TFE33 lends support to
the generality of our results.
In conclusion, the novel kinetic probe developed in the
present work allows a direct comparison to be made between
rate constants for a fundamental SN1-like reaction in zeolites
and rate constants for the same reaction in solution. The
results illustrate that the internal ionizing power of the alkali
metal cation exchanged zeolites depends strongly on the
nature of the counterion and the presence of water, and that
zeolites possess a strong ionizing ability that is similar to
solvents such as TFE and methanol/water mixtures. Further
studies into the ionizing power of zeolites are currently in
progress, especially with regard to the effect of probe
structure and the nature of the leaving group.
Acknowledgment. This work was supported by the
Natural Sciences and Engineering Research Council of
Canada (NSERC) and Dalhousie University. We thank
NSERC for a PGS (W.O.) and a WFA (F.L.C.).
(30) Liu, X. S.; Iu, K. K.; Thomas, J. K. J. Phys. Chem. 1989, 93, 4120-
4128.
(31) Ramamurthy, V.; Sanderson, D. R.; Eaton, D. F. Photochem.
Photobiol. 1992, 56, 297-303.
(32) Sarkar, N.; Das, K.; Nath, D. N.; Bhattacharyya, K. Langmuir 1994,
10, 326-329.
(33) Dutta, P. K.; Turbeville, W. J. Phys. Chem. 1991, 95, 4087-4092.
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